Or at least that’s what the first-term Democratic incumbent-turned-underdog is betting on.

Trailing in the polls and tethered to a president uniformly expected to lose her state, McCaskill’s chances at returning to Washington for another six-year stint may hinge on how nasty the GOP primary fight gets and who survives.

“Without question, the bloodier it gets, the better it is for Sen. McCaskill,” said Richard Martin, McCaskill’s 2006 campaign manager. He pointed to little-noticed Nebraska state Sen. Deb Fisher’s surprise triumph in that state’s May Republican primary: She was the beneficiary of two more widely known candidates who spent weeks pummeling one another.

“I think that could very well happen here,” Martin said.

Powered by a flood of advertising that began last October, wealthy businessman John Brunner recently emerged as a slight favorite over former state Treasurer Sarah Steelman and Rep. Todd Akin.

But that was before Steelman or Akin made a sustained play on the airwaves. The dynamic changed Monday when Steelman launched a lighthearted but pointed spot underlining Brunner’s past $10,000 donation to the Humane Farming Association. She dubbed the organization “an extreme animal-rights group that was founded to give farm animals rights.”

“They worry about her feelings,” a narrator says as a grazing cow slides into frame. “What’s next? Therapy?”

Brunner’s camp moved swiftly to denounce the ad, releasing a statement by the candidate’s daughter, who took responsibility for the charitable donation.

“If she wants to run an ad attacking a decision my daughter made,” Brunner said in an interview before briefly pausing, “This is not the problem we have in Missouri.”

Steelman, whose strategy is to position herself as a champion of rural Missouri against two St. Louis-centered candidates, accused Brunner of going negative first.

“John Brunner has been attacking me for weeks now. He’s spent $3 million, been on TV for 10 months. The last three weeks he has continually attacked me on things that are not true,” she said. “I’m simply pointing out who I am, where I came from. I am highlighting the fact that he’s trying to be something he’s not.”

It’s true that Brunner went up with the first comparative commercial of the primary at the end of June, slapping both Steelman and Akin as politicians with debt-ridden fiscal records. During the closing moments of the spot, he morphed his two GOP rivals into McCaskill and President Barack Obama under the headline “More Spending, More Debt. Sound Familiar?”